This invention pertains to steam and fuel oil control and purge valves, such as are used in the firing of industrial oil burners and auxiliary or igniter burners associated with large utility burners, and in particular to a sealing arrangement for use with, and in combination with such valves, and to a novel valve seat for such sealing arrangements. Valves of the aforesaid type must satisfy specific regulatory leakage specifications, Factory Mutual in particular, and have a limited acceptance leakage. The specifications also require a Proof of Closure (POC) switch. The basic concept is that, as the valve begins to close, oil flow stops, the valve continues closed tripping the POC, and finally ends up making the metal to metal seal. When opening, the valve piston begins to move, causing the POC to trip prior to any oil flow, and travel continues to the full open position. The POC switch must trip going closed after oil flow stops, and before going metal to metal, the POC switch must trip going open before oil flow starts again. This process cannot be satisfied if the seal leakage exceeds the allowable.
Valves of the aforesaid type commonly comprise a housing having a pair of chambers formed therein, means for admitting fuel into one of the chambers, means for discharging fuel from the other of said chambers, means for admitting steam into the other chamber, and valving means interposed between the chambers. The valving means is operative (a) for effecting communication between the chambers, and (b) for closing off communication therebetween, and comprises a valve seat-and a piston movable onto and from the seat, for closure of one of the chambers from the other, and for opening the chambers onto each other. Typically, the piston carries an annular seal in a recess provided therefor.
Exemplary of such an aforesaid steam and fuel oil control and purge valve is U.S. Pat. No. 4,146,056, bearing the same title, and issued to Bascom F. Buchanan, on Mar. 27, 1979. As noted, the patented valve has a translating piston which enters an orificed insert, the latter having an annular land, and the piston has a tapered flange or skirt which seats against the land. The piston-carried seal sealingly engages the wall of the orifice to prohibit inter-chamber communication. The seal is formed of plastic, i.e., virgin tetrafluoroethylene, and upon exposure to elevated temperature, exhibits creep. An option for the prior art valve was a reinforced tetrafluoroethylene. However, this material reacted in the same, unfavorable way to temperature and creep as virgin tetrafluoroethylene. The plastic moves away from the load; the load is created due to the interference fit between the seal outside diameter and the orifice wall. This is a natural response to load over time, and is accelerated by the temperature exposure. The creep of the plastic manifests itself by an attempt to fill the clearance gap between the piston and the wall of the orifice and, as a result thereof, the plastic displaces up and down the orifice wall. Subsequently, when the valve is cooled back to ambient temperatures, the plastic shrinks back toward the carrying piston. Therefore, the outer diameter of the seal becomes smaller. The effect results in a loss of the interference fit between the seal and the wall of the orifice; fluid flows freely past the seal and through the clearance fit which the shrunk or contracted seal presents.